1. Field Of The Invention
The invention relates to a novel process for the asymmetric hydrogenation of furoimidazole derivatives of the general formula: ##STR3## wherein R.sub.1 denotes a protective group which can be removed by methods known in the art and R.sub.2 represents hydrogen or a protective group which can be removed by methods known in the art, in the presence of a homogeneous catalyst with hydrogen to give the corresponding diastereomeric dihydrofuroimidazole derivatives of the general formula: ##STR4## wherein R.sub.1 and R.sub.2 have the above-mentioned meaning.
The dihydrofuroimidazoles of the general formula II are important intermediates in the synthesis of (+)-biotin, an essential vitamin for humans which is also termed vitamin H. (+)-Biotin is also used as a pharmaceutical for the treatment of dermatosis or as a feed additive having growth-promoting activity for farm animals.
2. Background Art
The majority of the known (+)-biotin syntheses pursue the aim of separating appropriate precursors via some highly laborious racemate cleavage methods using some highly expensive cleavage agents and using the resulting diastereomers to further pursue the (+)-biotin synthesis (cf., for example German Patent No. 2,058,248). According to European Published Patent Application No. 273,270, the introduction of the relevant optically active center, that is, the 3aS and 6aR positions of the biotin ring structure, was achieved for the first time via an asymmetric hydrogenation of corresponding furoimidazole derivatives using a classical hydrogenation catalyst, such as, rhodium on aluminum oxide. This prior art process was not completely satisfactory with respect to the attainable yield of the desired diastereomers.